In many situations, a signal supplied from an output of one circuit to an input of a second circuit may have to be at so high a current level to meet the demands of the second circuit that the first circuit is detrimentally affected. To alleviate this problem, an impedance buffer is typically placed between the output of the first circuit and the input of the second circuit. The impedance buffer has a high input impedance so as to draw little current from the first circuit but a low output impedance that enables the current level of the signal from the first circuit to be raised high enough to meet the requirements of the second circuit without harming the first. Often, the impedance buffer also acts as a voltage follower in that it changes the voltage level of the signal transmitted between the circuits by a known amount.
FIG. 1 illustrates a known bipolar impedance buffer for converting an input signal V.sub.I at a low current level into an output signal V.sub.O at a high current level. Output V.sub.O is at the same voltage level as input V.sub.I here. However, a conventional voltage-following element might be included at any of several points in this buffer so as to cause output V.sub.O to differ from input V.sub.I by a known voltage.
In this buffer, input V.sub.I is supplied to the base of an NPN input transistor Q1 whose collector is connected to a source of a high supply voltage V.sub.CC and whose emitter is connected through a node N1 to the emitter of a diode-connected NPN transistor QJ. The interconnected base and collector of transistor QJ are connected to the emitter of an intermediate NPN transistor QN whose base and collector are connected together through a node N2 to the base of an NPN output transistor QO. Its emitter supplies output V.sub.O while its collector is connected to the V.sub.CC supply. The interconnected base and collector of transistor QN are also connected to the collector of a PNP transistor QP whose emitter is connected to the V.sub.CC supply. A bias voltage V.sub.B is supplied to the base of transistor QP to make it conduct a current I.sub.p. A current source 5 is ccnnected between node N1 and a source of a low supply voltage V.sub.EE. Another current source 6 is connected between the V.sub.EE supply and the emitter of transistor QO.
When input V.sub.I changes rapidly, the voltage at the QP collector also changes rapidly. This change is partially transmitted across the QP base-emitter junction due to internal device parasitics and resistance in the V.sub.B line and causes a transient change in the current I.sub.p. In turn, this causes a transient error in output V.sub.O. After a while, current I.sub.p settles back to its nominal value. However, the settling time is relatively long because PNP base-emitter capacitances, particularly the QP base-emitter capacitance, are typically very large. Output V.sub.O thus takes a relatively long time to settle back to its nominal value. This is disadvantageous in high-speed applications.